The present invention relates to an anisotropic diffusing film which is useful for assuring a uniform light emission on a display device (a flat-type or surface display device), a method of producing the same, and a display device utilizing the film. More particularly, the invention relates to an anisotropic diffusing film which is useful for transmission type (mode) or reflection type (mode) liquid crystal display or projection television.
Among surface display devices (illumination devices) such as a liquid crystal display device, there is known a backlight type (transmission type) display device comprising a fluorescent tube disposed behind of a liquid crystal display module. Also is known a reflection type display device comprising a light reflecting layer formed on the back of a liquid crystal display module so that the light incident on the front surface is reflected by the light reflecting layer. In such display devices, a diffuser or diffusing film is frequently used to diffuse the light from the fluorescent tube or the reflected light from the light reflecting layer to assure a uniform luminance. As the diffusing film, a diffusing film comprising a polycarbonate or polyester base film which is transparent and highly heat-resistant, and containing refractive microfine particles (resin beads) or light-transmitting inorganic microfine particles incorporated therein by coating or containing is generally employed.
Recent years have seen an increasing demand for such diffusing films as the diffusing film for the backlight component of the backlight type liquid crystal display device. The diffusing film for backlight use is usually interposed between the backlight (cold cathode tube) and the liquid crystal layer to homogenize the light emitted from the cold cathode tube. However, when the diffusing of light is too large, no sufficient emission luminance can be obtained. Therefore, an optical element such as a prismatic lens is interposed between the diffusing film (diffuser) and the liquid crystal layer to thereby refract the diffused light so that the light will be incident perpendicularly on the liquid crystal display surface, thus upholding the luminance.
As the surface display device [i.e., a display device the image display area of which is a flat surface (a flat type display device)], the device illustrated in FIG. 4 is known. This device comprises a surface display module 45 (particularly a transmission type liquid crystal display module) and at least one fluorescent discharge tube (cold cathode tube) 41 which is adapted to illuminate the module from its back side. Disposed on the back side of the fluorescent discharged tube 41 is a reflector 42 for reflecting the light advancing toward the back side. Moreover a diffuser 43 for diffusing light to uniformly illuminate the module 45 is interposed between the fluorescent discharged tube 41 and the module 45 and a prism sheet 44 is disposed on the module side of the diffuser 43. This surface display module 45, in the case of a liquid crystal display module, comprises a first polarizing film 46a, a first glass substrate 47a, a first electrode 48a on the glass substrate, a first alignment layer 49a on the electrode, a liquid crystal layer 50, a second alignment layer 49b, a second electrode 48b, a color filter 51, a second glass substrate 47b, and a second polarizing film 46b as successively built up (laminated) in the order mentioned. In such a display device, the display module can be directly illuminated from the back side by the built-in fluorescent tube (cold cathode tube) 41. However, an uneven emission (luminance) distribution in the direction normal to the longitudinal axis of the fluorescent tube is inevitable, causing a streak pattern to appear, although the emission distribution in the longitudinal direction of the fluorescent tube is uniform.
Further, there is known the device constructed by using the backlight unit including a light guide illustrated in FIG. 5 as the backlight system of the surface display device of FIG. 4. This backlight unit has a fluorescent tube (cold cathode tube) 51 and a reflector member 55 disposed in parallel with the fluorescent tube, with a light guide 54 having a diffuser 53 at top and a reflector 52 at bottom being disposed in the direction of light emission from the fluorescent tube. The lower part of the light guide 54 is inclined so that the light from the fluorescent tube can be reflected in an upward direction. The light emerging in the direction of the top of the light guide 54 is diffused by the diffuser 53 and incident on the surface display module (not shown) constructed (laminated) on the diffuser 53.
When such a backlight unit is used, in contrast to the backlight unit or component of FIG. 4, the emission distribution may appear uniform over the surface but a detailed observation of the emission distribution reveals that the distribution is still not as uniform as desired. Thus, as shown in FIGS. 6 and 7, the emission distribution (luminance distribution) in the longitudinal (axial) direction (x-direction) of the fluorescent tube (cold cathode tube) 51 is small as it is the case in the device of FIG. 4 but the emission from the fluorescent tube (cold cathode tube) in the y-direction which is normal to the x-direction is repeatedly reflected by the reflector 52 and advances in the z-direction (the direction in which the liquid crystal display module is disposed) which is perpendicular to the xy plane so that the emission distribution (luminance distribution) in the y-direction is distorted (in a zigzag pattern), thus failing to assure sufficient uniformity.
Thus, in the usual backlight type display device, the emission distribution (luminance distribution) in the direction normal to the longitudinal direction (X-direction) of the fluorescent tube is not uniform and a streak-like directionality (linear dark areas) is produced in the emission distribution. Moreover, even when a diffusing film containing microfine particles is used, it is inevitable from its isotropy of light diffusing that the luminance in a certain direction (the direction of disposition of the fluorescent tube, the streaking direction, X-direction) is unduly lowered.
Japanese Patent Application Laid-Open No. 142843/1999 (JP-A-11-142843) describes a technology such that a dot pattern for scattering light is formed in rows perpendicular to the light source on the surface of the light guide. However, even with this contrivance, linear dark areas (a streak pattern) are observed in the direction of disposition of the fluorescent tube.
Japanese Patent Application Laid-Open No. 261171/1995 (JP-A-7-261171) describes a reflection type liquid crystal display device comprising a pair of glass substrates, an electrode formed on each of the opposed surfaces of the glass substrates, a liquid crystal sealed interposed between the electrodes, and a polarizing film formed on (laminated) the outer surface of the external one of the pair of glass electrodes, with a light-scattering layer composed of two or more kinds of resins differing from each other in the index of refraction and forming mutually segregated (separated) phases being disposed on the surface of the polarizing film. In this literature, it is mentioned that the polarizing film is coated or printed with a mixture of two or more kinds of resins in a solvent to form the light-scattering layer.
Furthermore, as a reflection type liquid crystal display device (or a reflection type liquid crystal module), the device (or module) illustrated in FIG. 8 is also known. This reflection type display module comprises a pair of glass substrates 81a, 81b, a pair of electrodes 82a, 82b formed on the opposed surfaces of the glass substrates, and a liquid crystal 87 interposed between the electrodes forming the pair, the electrode 82a formed on the glass substrate 81a on the back side constituting light-reflective pixel electrodes and a color filer 84 being interposed between the glass substrate 81b on the front side and the electrode 82b. In addition, a phase difference layer 86 is constructed (laminated) through a polarizing layer 85 on the front surface of the glass substrate 81b on the front side. In this reflection type liquid crystal display module, a diffuser 83 is laminated on the front surface (the front surface of the phase difference layer 86) to constitute a reflection type liquid crystal display device. Since, in the reflection type liquid crystal display device, one polarizing layer 85 is situated on the front side of the liquid crystal cell, unlike in the transmission type display device having a built-in lamp (the backlight type liquid crystal display device), the incident light which is incident on the front surface of the device (external light) is diffused by the diffuser 83 and enters into the liquid crystal cell and then is reflected by the reflective electrode (reflector) 82a within the liquid crystal cell and diffused by the diffuser 83. Therefore, the data or image displayed on the display module can be visually recognized from any angle without loss of luminance without the provision of a lamp (light) but utilizing external light.
In the reflection type liquid crystal display device, however, if the light diffusing power or ability of the diffuser is too great, the incident light and the reflected light are randomly reflected in a large measure by the diffuser so that the clarity of displayed data is sometimes sacrificed.
Meanwhile, Japanese Patent Application Laid-Open No. 314522/1992 (JP-A-4-314522) describes an anisotropic light-scattering material comprising a transparent matrix and a (particulate) transparent substance which is morphologically anisotropic and differing in the index of refraction from the transparent matrix as uniformly dispersed in the matrix in a positional relation shifted in an orderly and mutually parallel manner. In this literature, it is disclosed that an aspect ratio of the anisotropic form is preferably 15 to 30, and the dimension of the minor axis is 1 to 2 xcexcm. Concretely, a low-density polyethylene having a low melting point is employed as the transparent matrix resin, and a polystyrene or a styrene-acrylonitrile copolymer which has a higher melting point than that of the above polyethylene is employed as the transparent substance. The anisotropic light-scattering material is produced by extruding a composition obtained by melt-kneading the transparent matrix resin and the transparent substance and cooling the molten resin which is extruded in the form of a sheet under a large draft applied in the direction of extrusion. The anisotropic light-scattering material is employed as a lenticular lens for the projection television screen.
However, as described in FIGS. 3 to 6 of the literature, when Fy(xcex8) represents a light-scattering characteristic (intensity) at a scattering angle xcex8 of a scattering light on a plane which is perpendicular to the major axis of the dispersed particle and Fx(xcex8) represents a light-scattering characteristic (intensity) at a scattering angle xcex8 of a scattering light on a plane which is parallel to the major axis of the dispersed particle, the ratio of Fy(xcex8) to Fx(xcex8) at the scattering angle xcex8=4xc2x0 is Fy(4xc2x0)/Fx(4xc2x0)=about 2. Thus, anisotropy of the anisotropic light-scattering material is still inadequate. Further, the clarity of displayed data is insufficient.
Further, in the process for producing a sheet described in the literature, a sheet can not be steadily produced for a long time since the transparent material in anisotropic form forming the dispersed phase is attached to a wall of a die used in extruding, or to a roll used in drawing. A molten resin which is extruded in the form of a sheet is cooled with stretching and drawing so that a sheet width is remarkably reduced and thicknesses at both edges of the sheet tend to be thick after stretching, thus the useful rate (yield) of the sheet is deteriorated.
The anisotropic light-scattering material (sheet) is wound in the form of a roll or cut in suitable size and laminated for preservation. Further, as to the supply form for using as a part, about one hundred of the cut sheets are laminated. However, it is possible that the wound or laminated sheets stick to each other (blocking) in case of standing for a long term or under the circumstances of high temperature and humidity.
The anisotropic scattering material is sometimes bent slightly when incorporated in a display device. However, attention should be given to handling of the anisotropic light-scattering material since the material tends to receive a fold and a flaw thereon by slightly bending.
Though the display device is required to have high heat resistance in order to use under a wide variety of conditions, the anisotropic scattering material is inadequate in heat resistance since it contains a dispersing agent which begins to soften at a relatively low temperature.
It is, therefore, object of the present invention to provide a laminated film assuring a uniform surface emission with close tolerances and capable of being steadily produced for a long term, a method of producing the same, and a display device (particularly a liquid crystal display device) utilizing the film.
It is another object of the present invention to provide a laminated film having excellent storage stability, heat resistance, handling and the like, a method of producing the same, and a display device (particularly a liquid crystal display device) utilizing the film.
It is still another object of the present invention to provide a laminated film giving a uniform surface emission easily even if the light from a light source has an anisotropy of emission distribution (distribution of luminance) without compromise in luminance, a method of producing the same and a display device (particularly a liquid crystal display device) utilizing the film.
It is another yet object of the present invention to provide a laminated film having a good anisotropy of light scattering despite its high transparency, a method of producing the same, and a display device (particularly a transmission type liquid crystal display device) utilizing the film.
It is further object of the present invention to provide a reflection type liquid crystal display device which upholds the clarity of displayed data and has a strong display directionality.
It is another object of the present invention to provide a lenticular lens having a good anisotropy of light scattering as well as high heat resistance.
The inventors of the present invention did much research to accomplish the above objects and found that, a film comprises an anisotropic light-scattering layer having a continuous phase and a dispersed phase having the specific structure, so that extremely high optical anisotropy is impaired to a scattering light, and by using the film, an anisotropy (directionality) of the emission distribution of a light source such as a fluorescent tube can be relaxed without deteriorating luminance and further by laminating a transparent resin layer on the anisotropic light-scattering layer, not only the stability of the film producing step but also handling of the film, storage stability, heat resistance and the like are improved. Furthermore, the laminated film comprising the anisotropic light-scattering layer and the transparent resin layer is monoaxially stretched, so that the higher (sharper) optical anisotropy is impaired to the film with compared to the case of monoaxially stretching a film comprising the anisotropic light-scattering layer alone.
That is, a laminated film of the present invention comprises an anisotropic light-scattering layer and a transparent resin layer laminated on at least one side (in particular, both sides) of the light-scattering layer. The light-scattering layer is composed of a continuous phase and a particulate dispersed phase which have the different refraction indexes from each other, the mean aspect ratio of the particulate dispersed phase is more than 1 and the major (longitudinal) axis of the particulate dispersed phase is oriented in a direction. The transparent resin layer may be composed of the same kind of resins as one constituting the continuous layer. The laminated film may be capable of diffusing incident light in its advanced direction and having a light-scattering characteristic F(xcex8) satisfying the following expression representing the relation between the angle of light scattering xcex8 and the intensity of scattered light F over a range of xcex8=4 to 30xc2x0:
Fy(xcex8)/Fx(xcex8) greater than 5
wherein Fx(xcex8) represents the light-scattering characteristic in the direction of the major axis of the particulate dispersed phase and Fy(xcex8) represents the light-scattering characteristic in the direction normal to the major axis of the particulate dispersed phase.
The mean aspect ratio of the particulate dispersed phase may be 5 to 1,000. The mean dimension of the minor axis of the particulate dispersed phase may be 0.1 to 10 xcexcm. The weight ratio of the continuous phase to the particulate dispersed phase may be [the continuous phase/the particulate dispersed phase]=99/1 to 30/70. The ratio in thickness of the anisotropic light-scattering layer (1) to the transparent resin layer (2) may be [the light-scattering layer/the transparent resin layer]=50/50 to 99/1, and the total thickness of the film may be 6 to 600 xcexcm. The total light transmittance is usually not less than 85%.
The laminated film can be produced by coextruding a mixture of resins constituting the continuous phase and the dispersed phase, and a resin constituting the transparent resin layer to form a film under drawing. Moreover, the laminated film can be produced by coextruding a mixture of resins constituting the continuous phase and the dispersed phase, and a resin constituting the transparent resin layer, solidifying the extruded product and monoaxially stretching (e.g., monoaxially stretching by roll-calendering) the solidified product. The film may be monoaxially stretched at a temperature higher than the melting point or glass transition temperature of the resin constituting the dispersed phase.
The present invention also includes a display device comprising an image display module, a tubular light projector means disposed behind said image display module and adapted to project light to said module and the laminated film as disposed forwardly of said light projector means. The laminated film is disposed with the main axis of the particulate dispersed phase oriented in the longitudinal direction of said projector means. Further, the present invention includes a reflection type liquid crystal display device comprising the laminated film disposed, and a lenticular lens for projection television comprising the laminated film.
Throughout this specification, the term xe2x80x9cfilmxe2x80x9d is used without regard to thickness, thus meaning a sheet as well.